Upshifting from a lower speed ratio to an upper speed ratio in an automatic transmission involves releasing a fluid operated torque transmitting element associated with the lower speed ratio while engaging a fluid operated torque transmitting element associated with the upper speed ratio. The torque transmitting elements are referred to herein as clutches The element to be released is referred to as an off-going clutch, and the element to be engaged is referred to as an on-coming clutch.
The engagement of the on-coming clutch is controlled via regulation of the fluid pressure supplied thereto, referred to herein as the on-coming pressure. The commanded pressure includes a clutch dependent component and a torque dependent component. The clutch dependent component is an empirically derived pressure required to overcome a preloaded clutch return spring which biases the clutch toward disengagement and is referred to herein as the return spring pressure. The torque dependent component is scheduled based on an estimation of the input torque, the mechanical gain of the clutch and the speed ratio with which the clutch is associated.
Similarly, the release or disengagement of the off-going clutch may be electronically controlled via regulation of the off-going pressure, as in the illustrated embodiment. Alternatively, the release of the off-going clutch may be controlled mechanically with an overrunning or one-way clutch mechanism.
An upshift is fundamentally separable into fill, torque and inertia phases. In the fill phase, the on-coming clutch is filled with fluid in preparation for engagement. In the torque phase, the on-coming pressure is progressively increased to increase the on-coming clutch torque capacity while the off-going pressure is progressively released to reduce the off-going clutch torque capacity. The initiation of the inertia phase is marked by a consequent slippage of the off-going clutch (and corresponding reduction of the transmission input speed), the shift being complete when the on-coming clutch is fully engaged.
Accurate control of the on-coming torque capacity during the torque phase requires accurate knowledge of the on-coming fill time and the return spring pressure. Both the fill time and the return spring pressure are empirically determined and subject to variations associated with age and manufacture. In turn, successful closed-loop control of the on-coming clutch in the inertia phase depends on whether the on-coming torque capacity at the initiation of the inertia phase is sufficient to carry the input torque. Accurate knowledge of the fill time and on-coming return spring pressure are therefore essential to the achievement of consistent high quality shifting.
Analysis of the on-coming clutch fill time and return spring pressure reveals that variation of either parameter produces variation in the torque phase of the shift. Consequently, a detected deviation from a desired condition during the torque phase may be due to variability in one or both of the empirically derived on-coming clutch control parameters.